It is known to add various chemicals to waste water in settling tanks in which the chemicals act to flocculate the contaminants so that they can settle for collection at the bottom of the settling tank leaving clarified water to be removed from the top of the settling tank. Conventionally this takes several hours of settling so that the tanks must be very large in order to accommodate a significant flow through.
One arrangement intended to allow settling of the materials within a few minutes thus reducing the size of the equipment necessary is shown in the prior U.S. Pat. No. 4,710,290 of one of the inventors herein Briltz. This arrangement has achieved some success and provides a tubular mixing chamber into which the waste water is injected and within which is provided a plurality of chemical injectors at spaced positions along the length of the duct. Also along the duct is provided a plurality of mixing devices which rotate within the duct so as to ensure that the chemicals are properly mixed in the water. It will be appreciated that the mixing is highly important since it is necessary for the chemicals to mix intimately with the contaminants within the water to ensure that the flocculation occurs quickly in order to achieve the short dwell time within the system.
Downstream of the mixing chamber is provided a settling tank which can be very small allowing a dwell time of only a few minutes in view of the fact that the highly effective mixing has previously occurred so that the flocculation occurs immediately the materials enter the settling tank.